# std::size_t

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< cpp‎ | types

Defined in header `<cstddef>` |  |
Defined in header `<cstdio>` |  |
Defined in header `<cstdlib>` |  |
Defined in header `<cstring>` |  |
Defined in header `<ctime>` |  |
Defined in header `<cuchar>` |  | (since C++17)
Defined in header `<cwchar>` |  |
typedef /* implementation-defined */ size_t; |  |

`std::size_t` is the unsigned integer type of the result of the following operators:

- `sizeof`

`sizeof...` `alignof` | (since C++11)

If a program attempts to form an oversized type (i.e., the number of bytes in its object representation exceeds the maximum value representable in `std::size_t`), the program is ill-formed.

The bit width of `std::size_t` is not less than 16. | (since C++11)

### Notes

`std::size_t` can store the maximum size of a theoretically possible object of any type (including array). On many platforms (an exception is systems with segmented addressing) `std::size_t` can safely store the value of any non-member pointer, in which case it is synonymous with std::uintptr_t.

`std::size_t` is commonly used for array indexing and loop counting. Programs that use other types, such as unsigned int, for array indexing may fail on, e.g. 64-bit systems when the index exceeds UINT_MAX or if it relies on 32-bit modular arithmetic.

When indexing C++ containers, such as std::string, std::vector, etc, the appropriate type is the nested type `size_type` provided by such containers. It is usually defined as a synonym for `std::size_t`.

It is unspecified whether the declaration of `std::size_t` is available in any other standard library header. An implementation may avoid introducing this name even when the standard requires `std::size_t` to be used.

The integer literal suffix for `std::size_t` is any combination of `z` or `Z` with `u` or `U` (i.e. `zu`, `zU`, `Zu`, `ZU`, `uz`, `uZ`, `Uz`, or `UZ`). | (since C++23)

### Example

Run this code

    #include <array>
    #include <cstddef>
    #include <iostream>

    int main()
    {
        std::array<std::size_t, 10> a;

        // Example with C++23 std::size_t literal
        for (auto i = 0uz; i != a.size(); ++i)
            std::cout << (a[i] = i) << ' ';
        std::cout << '\n';

        // Example of decrementing loop
        for (std::size_t i = a.size(); i--;)
            std::cout << a[i] << ' ';
        std::cout << '\n';

        // Note the naive decrementing loop:
        //  for (std::size_t i = a.size() - 1; i >= 0; --i) ...
        // is an infinite loop, because unsigned numbers are always non-negative
    }

Output:

    0 1 2 3 4 5 6 7 8 9
    9 8 7 6 5 4 3 2 1 0

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 1122 | C++98 | `std::size_t` was circularly defined[1] | it is implementation-defined
CWG 1464 | C++98 | object size might be not representable in `std::size_t` | such type is ill-formed

1. ↑ The definition of `std::size_t` was exactly the same as the definition of `size_t` in C, which is “the result type of `sizeof`”. There is no circular definition in C because the result type of `sizeof` in C is an implementation-defined unsigned integer type.

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 6.8.4 Compound types [basic.compound] (p: 79-80)

  - 7.6.2.5 Sizeof [expr.sizeof] (p: 136)

  - 7.6.2.6 Alignof [expr.alignof] (p: 136)

  - 17.2.4 Sizes, alignments, and offsets [support.types.layout] (p: 504-505)

- C++20 standard (ISO/IEC 14882:2020):

  - 6.8.3 Compound types [basic.compound] (p: 75-76)

  - 7.6.2.5 Sizeof [expr.sizeof] (p: 129-130)

  - 7.6.2.6 Alignof [expr.alignof] (p: 130)

  - 17.2.4 Sizes, alignments, and offsets [support.types.layout] (p: 507-508)

- C++17 standard (ISO/IEC 14882:2017):

  - 6.9.2 Compound types [basic.compound] (p: 81-82)

  - 8.3.3 Sizeof [expr.sizeof] (p: 121-122)

  - 8.3.6 Alignof [expr.alignof] (p: 129)

  - 21.2.4 Sizes, alignments, and offsets [support.types.layout] (p: 479)

- C++14 standard (ISO/IEC 14882:2014):

  - 3.9.2 Compound types [basic.compound] (p: 73-74)

  - 5.3.3 Sizeof [expr.sizeof] (p: 109-110)

  - 5.3.6 Alignof [expr.alignof] (p: 116)

  - 18.2 Types [support.types] (p: 443-444)

- C++11 standard (ISO/IEC 14882:2011):

  - 5.3.3 Sizeof [expr.sizeof] (p: 111)

  - 5.3.6 Alignof [expr.alignof] (p: 116)

  - 18.2 Types [support.types] (p: 454-455)

- C++03 standard (ISO/IEC 14882:2003):

  - 5.3.3 Sizeof [expr.sizeof] (p: 79)

- C++98 standard (ISO/IEC 14882:1998):

  - 5.3.3 Sizeof [expr.sizeof] (p: 77)

### See also

ptrdiff_t | signed integer type returned when subtracting two pointers (typedef)
offsetof | byte offset from the beginning of a standard-layout type to specified member (function macro)
integer literals | binary,(since C++14) decimal, octal, or hexadecimal numbers of integer type
C documentation for size_t

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